In many experimental research and industrial applications equipment and methods are used that are adversely affected by vibration. Vibration may be intrinsically caused by the equipment and methods, or the vibration may be transferred to the equipment from the surrounding environment. As such, it is desirable in these circumstances to have a low vibration mount surface and mount components for mounting such sensitive equipment. One example of an optical structure with an optical mount surface is an optical table which is typically used for mounting optical equipment as well as other equipment that is sensitive to vibration. In order to reduce vibration transferred to an optical table, most optical tables are equipped with vibration isolators which reduce vibration transmitted to the table from the floor upon which the table rests.
The vibration isolators may be assembled to the table at predetermined locations to optimize damping of the vibrational modes of the optical table. Often, one or more vibration isolators are positioned between a coupling surface of the optical table and the one or more table supports or legs, thereby passively limiting the transmission of vibrational influences from the environment to the devices supported on the coupling surface of the table. Exemplary passive vibration isolators include fluid bladders, springs, shocks, foams, and the like.
An optical table itself, however, has its own natural frequencies and corresponding flexural vibration modes that can be easily excited by residual vibration coming through the isolators or by other sources such as acoustical excitation, air turbulence and dynamic forces generated by the payload equipment installed on the optical table. The main flexural vibration modes usually have a global character, which means that an excitation at any point of the table generates a vibration pattern encompassing the whole optical table structure. These natural vibrations are only very lightly damped, in general, and therefore can reach high amplitudes unless special damping means are introduced into the optical table structure. Some special damping means may include passive dampers that may be specifically tailored to natural frequencies of the optical table.
Passive dampers of various designs are widely used in construction of optical tables. The “Shock and Vibration Handbook”, ed. By C. M. Harris, 4th edition, 1996; 5th edition, 2001, Ch. 37, provides a survey and a classification of dampers or damping treatments. According to this reference, known types of damping treatments include free-layer damping treatments, where the energy is dissipated by means of extensional deformation of a damping layer (made of visco-elastic material) induced by flexural vibration of the base structure. Also included are constrained-layer damping treatments, where the constraining layer helps induce relatively large shear deformations in the visco-elastic layer in response to flexural vibration of the base structure, thereby providing more effective energy dissipation mechanism. Also included are integral damping treatments, including use of damped laminated sheets and/or damped joints in the construction assembly and tuned passive dampers, which are essentially mass-spring systems having resonances which are matched or tuned to one or more resonance frequencies of the base structure. The application of the tuned damper replaces the resonance peak of the base structure, typically, by two peaks of lesser amplitude. Finally, damping links, i.e., visco-elastic elements joining two parts of the structure that experience large relative motion in the process of vibration are disclosed.
However, even with such passive damping equipment installed, an optical table or other optical structure may have vibration characteristics different than an analytical model that was used to tune such damping equipment. Additionally, differing payload configurations may create differing harmonic frequencies as well as differing nodes and anti-nodes in an optical table or other optical structure. Also, there is a growing demand for high precision and high throughput capabilities in the optoelectronics and semiconductor industries, as well as similar needs for modern scientific experimental instruments. These needs require higher damping performance of optical structures such as optical tables and optical components that may be mounted to optical mount surfaces of the optical tables.
In light of the foregoing, there is an ongoing need for methods and devices configured to efficiently reduce vibration in an optical structure that arise from a variety of vibration sources. In addition, there is a need for vibration damping capability that may be moved about an optical mount surface of an optical structure such as an optical table or the like in order to position the damping capability in the location where it is needed most. What has also been needed are systems and methods for applying vibration damping capability directly to optical components which may be mounted onto an optical mount surface of an optical structure. Also, there is a need for control systems for active vibration damper assemblies that are adaptable to payload changes on an optical structure, can accommodate a wide variety of vibration magnitude variations, can be easily or automatically tuned and can be incorporated into active vibration damping systems which may be produced at a modest cost.